1. Field of the Invention
The present invention generally relates to a sensor for detecting the degree of deterioration of oil, and more particularly, to an oil deterioration sensor being capable of detecting deterioration of oil and/or an oil level by utilizing the fact that the sound velocity of an ultrasonic wave varies with the alkalinity of oil.
2. Description of the Prior Art
Lubrication functions of engine oil for automobiles deteriorate owing to the deterioration of the oil itself, to combustion products of a fuel, and to dust, moisture and abrasion powder contained in intake air. Therefore, oil itself requires oxidation stability, acid corrosion-resistance and the suppression of generation of water and bubbles. A base number (namely, a total base number) serving as an index concerning various additives, which indicates an oil characteristic, is also an index to total basic components or ingredients, which is represented by an amount of potassium hydroxide needed for the neutralization of basic components contained in the oil. This index is effective in evaluating or determining the degree of deterioration of oil.
It is known that, especially in the case of engine oil for automobiles, such a base number controls the acid neutralization, which is employed as an engine-oil characteristic, and the oil deterioration characteristics, such as cleanliness, of engine oil. Hitherto, regarding the measurement of the density of gasoline for automobiles, for example, the Japanese Unexamined Patent Publication (Kokai) No. 7-225228 has disclosed the techniques of measuring a delay time to the transmission of an ultrasonic wave signal from an ultrasonic sensor, and then calculating the density of a fuel from this delay time and finally distinguishing the property thereof from this density.
However, the detection of the degree of deterioration of engine oil by the methods of measuring a delay speed of propagation of an ultrasonic wave has never been conducted. Even if it is intended to adopt such conventional methods, in the case that the deterioration of engine oil is detected from the strength of a reflected wave, the strength of the reflected wave is reduced when air bubbles are mixed in the oil. Thus, a detection error is caused. Consequently, it is substantially impossible to adopt the aforementioned conventional methods. Further, the temperature of the oil rises after an engine is started. Thus, the sound velocity of an ultrasonic wave changes. Consequently, in the case of the aforementioned techniques using ultrasonic waves, there is the necessity for using a high-precision thermometer that excels in response and resolution. Moreover, the equipment cost rises. Thus, there are such problems for the practical application of such conventional techniques. Therefore, the development of a sensor which utilizes ultrasonic waves as a relatively simple or convenient means for measuring the degree of deterioration of engine oil and has high responsibility and is advantageous in terms of the cost, has been desired.